1. Field of the Invention
The present invention relates to a light emitting diode having light emitting cells with different sizes and a light emitting device having the same.
2. Discussion of the Background
A light emitting diode (LED), which is a photoelectric conversion device having a structure in which an N-type semiconductor and a P-type semiconductor are joined together, emits predetermined light through recombination of the electrons and holes. Such an LED is widely used for display elements and backlights. Further, LEDs have less electric power consumption and a longer lifespan as compared with conventional light bulbs or fluorescent lamps, so that their application areas have been expanded to the use thereof for general illumination while substituting for conventional incandescent bulbs and fluorescent lamps.
An LED is repeatedly turned on/off depending on the direction of current under an AC power source. Hence, when the LED is directly connected to an AC power source, the LED may not continuously emit light and may be easily damaged due to reverse current.
To solve such a problem, an LED capable of being connected directly to a high-voltage AC power source is disclosed in PCT Patent Publication No. WO 2004/023568(A1), entitled “LIGHT-EMITTING DEVICE HAVING LIGHT-EMITTING ELEMENTS” by SAKAI et al.
According to PCT Patent Publication No. WO 2004/023568(A1), LEDs are two-dimensionally connected in series on an insulative substrate such as a sapphire substrate to form LED arrays. Such two LED arrays are connected to each other in reverse parallel on the sapphire substrate. As a result, there is provided a single-chip light emitting diode capable of being directly driven by an AC power supply.
FIG. 1 is a view showing an arrangement of light emitting cells in a conventional LED.
Referring to FIG. 1, a conventional LED 10 performs light emitting operation by AC power supplied from a power supply 20 for supplying AC power.
The LED 10 comprises a plurality of light emitting cells 11 arranged in two parallel rows, where first and second rows are arranged to have polarities opposite to each other.
Hence, if AC power is applied from the power supply 20, current flows into the first row in positive voltage intervals such that the light emitting cells 11 in the first row emit light, and current flows into the second row in negative voltage intervals such that the light emitting cells 11 in the second row emit light.
Therefore, the first and second rows alternately emit light.
The respective light emitting cells 11 of the LED 10 are formed on one substrate through the same process. The respective light emitting cells 11 in the LED 10 are formed to be electrically separated from one another on the substrate and then connected electrically to one another by metal wires.
At this time, the respective light emitting cells 11 in the LED 10 have the same size. Hence, the respective light emitting cells 11 have almost the same turn-on voltage as shown in FIG. 2. When power is applied to respective light emitting cells, the turn-on voltage of each light emitting cell is determined depending on the current density of a corresponding light emitting cell, and the respective light emitting cells 11 in the LED 10 are formed on the one substrate through the same process to have the same size. For this reason, the respective light emitting cells 11 in the LED 10 have the same current density.
If AC with a frequency of 60 Hz is applied as the respective light emitting cells 11 in the LED 10 have the same turn-on voltage, the light emitting cells 11 are periodically turned on by the applied AC to thereby emit light.
That is, the respective light emitting cells 11 are turned on to emit light if the voltage applied by the AC with a frequency of 60 Hz is over the turn-on voltage, while the respective light emitting cells 11 suspend emitting light if the voltage applied by the AC with a frequency of 60 Hz is below the turn-on voltage.
At this time, as the respective light emitting cells 11 have the same turn-on voltage, all the light emitting cells 11 are turned on at a certain time. Then, the respective light emitting cells 11 stop emitting light at the instance when the voltage drops below the turn-on voltage after the light emitting cells 11 are turned on at the same turn-on voltage, which causes a flicker phenomenon to occur.
Such a flicker phenomenon may not be easily visible with the naked eyes. However, in the field of illumination using AC power with a plurality of light emitting cells provided, a stable light source is required in reducing the flicker phenomenon if possible.